A Reverb simulates the component of sound that results from reflections from surrounding walls or objects. It is in effect a room simulator. Some people think it's just a delay effect with some filters, but it's way more complex than that.

There are three possible reasons for adding reverb:

To restore the natural sound as the listener would expect to hear it. For example, a recording done in a very low-reverb studio might sound unnatural unless reverb is added.

To enhance the sound. For example, it is common to give vocal recordings more reverb than what would be considered natural. Reverb helps fill out the voice, giving it more "body" and is usually considered to be a flattering effect. Reverb can even help smooth minor vocal fluctuations so they aren't as obvious.

To create special effects such as dream sequences, etc.

Reverb controls

In Gain: sets the input gain of signal, in decibels (dB)

Reverb Mix: sets the reverb mix, in decibels (dB). At zero there is no effect (Dry). At 100% only the processed reverb signal (Wet) is produced.

Reverb Time: sets the reverb time, in milliseconds. Short reverb times simulate small room reflections. Use longer reverb time to simulate larger spaces